Lactation induces substantial changes in maternal calcium and bone metabolism, as the mother's skeleton serves as a major source of calcium in milk production. During lactation, there is a dramatic decline in bone mass and microstructure similar to postmenopausal bone loss, as both phenomena are associated with reduced estrogen levels and increased bone turnover. What is remarkable about lactation-induced bone loss is that bone mass and structure are rapidly restored after weaning. The mechanism that controls the switch from net resorption during lactation to net formation after weaning is not well understood. It has been postulated that the increase in estrogen levels after lactation contributes to bone recovery. However, when used to treat post- menopausal osteoporosis, estrogen replacement therapy (ERT) suppresses both bone resorption and formation, and thus does not restore the deteriorated skeleton in the manner that occurs in lactating women after weaning. During lactation, PTH-related protein (PTHrP) is secreted from the mammary gland into the blood stream. The increased circulating level of PTHrP contributes to the increased rate of bone resorption and bone loss during lactation. However, the endocrine function of PTHrP on mediating post-lactation bone recovery is unclear. A recent study showed that cessation of a 7-day infusion of PTHrP caused an abrupt rebound of bone formation activities. Given the fact that plasma PTHrP levels rapidly decrease post-lactation, we hypothesize that this change in circulating PTHrP plays an important role in stimulating post-lactation bone formation. The overall goal of this study is to define the role of estrogen and circulating PTHrP in the remarkable bone structure and strength recovery after lactation, which may provide new insights into therapeutic strategies for recovering the lost structural integrity associated with postmenopausal osteoporosis. In Aim 1, unique structural recovery mechanisms of replacing disconnected trabecular rods and perforated trabecular plates during weaning will be elucidated by using in vivo CT imaging and individual trabecular dynamics analysis, and these recovery mechanisms will be contrasted with those of ovariectomized (OVX) rats given ERT. In Aim 2, the role of the drop in PTHrP levels at weaning will be elucidated by simulating lactation- induced fluctuations in PTHrP and estrogen levels in an OVX model, and by preventing the drop in PTHrP that normally takes place at weaning in post-lactation rats. Results of this study will advance our understanding of systematic regulation of post-lactation bone recovery. These data will also drive our investigations of targeted bone formation by defining local signals that trigge the repair of structural deficits.